Flaviviruses are small, enveloped, positive-strand RNA viruses that are of concern in many medical and veterinary settings throughout the world. West Nile (WN) virus, for example, which is a member of the flavivirus family, is the causative agent of WN encephalitis, an infectious, non-contagious, arthropod-borne viral disease (Monath et al., “Flaviviruses,” In Virology, Fields (ed.), Raven-Lippincott, New York, 1996, pp. 961-1034). The virus has been found in Africa, western Asia, the Middle East, the Mediterranean region of Europe, and, recently, in the United States. Mosquitoes become infected with the virus after feeding on infected wild birds, and then transmit the virus through bites to humans, birds, and animals, such as horses, sheep, cattle, and pigs.
In 1999, twenty-five horses in New York with neurological symptoms were found to have WN virus infection. These horses presented with signs of ataxia, difficulty walking, knuckling over, head tilt, muscle tremors, and the inability to rise. Of these twenty-five horses, nine died or were euthanized, and virus, as well as virus-specific antibodies, were found in tissue samples from these horses. The sixteen surviving horses all recovered, and also developed WN virus antibody titers. Since then, increasing numbers of West Nile virus-infected horses have been confirmed.
Flavivirus proteins are produced by translation of a single, long open reading frame to generate a polyprotein, which undergoes a complex series of post-translational proteolytic cleavages by a combination of host and viral proteases to generate mature viral proteins (Amberg et al., J. Virol. 73:8083-8094, 1999; Rice, “Flaviviridae,” In Virology, Fields (ed.), Raven-Lippincott, New York, 1995, Volume I, p. 937). The virus structural proteins are arranged in the polyprotein in the order C-prM-E, where “C” is capsid, “prM” is a precursor of the viral envelope-bound M (membrane) protein, and “E” is the envelope protein. These proteins are present in the N-terminal region of the polyprotein, while the non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5) are located in the C-terminal region of the polyprotein.